Traditionally, in a wireless communication system, a radio base station, RBS, transmits downlink, DL, signals to user equipments, UEs, by transmitting the DL signals in the entire cell which is covered by the RBS. In other words, the power of a DL signal is transmitted in the entire cell independent of the location of the UE to which the DL signals are to be received, hereinafter called the receiving UE.
To be able to concentrate the power of DL signals in a direction from the RBS towards the receiving UE, a method called beam forming is used. For being able to use beam forming for DL signals, an RBS is equipped with multiple, spatially separated, transmission antennas. The antennas are individually supplied with a weighted amount of the DL signal in such a way that the individual signals, when transmitted from the RBS, experience constructive interference between each other at a transmission angle towards a receiving UE while at other angles the individual signals experience destructive interference. As a result, the overlapping total DL signal will be concentrated in the direction towards the receiving UE.
The advantages of beam forming are, among others, increased signal strength at the location of the receiving UE, This advantage comes as a result of the power concentration in the direction towards the receiving UE. As a result, the signal to interference ratio (SINR) at the receiving UE is improved.
Beam forming may also be used in the uplink. In this case the RBS is equipped with multiple, spatially separated reception antennas.
In traditional RBS transmission techniques, one beam is formed, which beam may be directed to cover the whole cell. Alternatively, the beam is narrower and it is electronically tilted such that it is directed towards the UE currently receiving the transmitted information. The beam may be directed in the azimuth direction or in the elevation direction or in a combination of azimuth and elevation direction.
In another RBS transmission technique called beam selection, the RBS has the ability to transmit a plurality of beams, the plurality of beams being directed, or beam formed, in different directions within the cell covered by the RBS such that each beam covers a part of the cell. The UEs present in the cell are instructed to recommend one of the plurality of transmitted beams for its reception of DL signals, and to report its recommendation to the RBS. The recommendation may be performed based on signal strength measurements performed by the UE.
When communicating between an RBS and UEs using beam forming techniques there is a problem that transmissions originating from the RBS and directed towards UEs may cause interference to other UEs that are communicating with a different RBS. In fact, it is possible that the gain on a system level obtained from using beam forming techniques increasing the received signal level may be lost due to a simultaneous increase in interference towards UEs in a neighboring cell.